Make routing all traffic over wireguard tunnel work

Fixes #26 (regression introduced in
126a0dad4b)

This is not an ideal solution, since it disables some features of the
firewall. Ideally, the mullvad configuration would be declaratively
managed and include a PostUp and PreDown command that adds routes to the
tunnel endpoint to the physical interface.

modules/default.nix

@@ -99,6 +99,10 @@ in
     # NixOS state version (see https://nixos.wiki/wiki/FAQ/When_do_I_update_stateVersion)
     system.stateVersion = "20.03";
 
+    # When this is set to true (default), routing everything through a
+    # wireguard tunnel does not work.
+    networking.firewall.checkReversePath = "loose";
+
     nix = {
       nixPath = [
         "/var/src" # pinned nixpkgs and configuration